GBBL capacitors are capable of satisfying the ever increasing demand for small, high capacitance ceramic capacitors due to the rapid miniaturization of modern electronics. GBBL capacitor dielectrics comprising conducting grains separated by an insulating layer at the grain boundaries are usually formed by a two step firing process in a reducing atmosphere. The insulating layer at the grain boundary is usually formed either by oxidation or by impregnation with a glassy mixture during a second firing stage. FIG. 1 is a scanning electron micrograph of a typical GBBL dielectric based on BaTiO.sub.3. Reference is made to S. Waku et al, "Classification and Dielectric Characteristics of the Boundary Layer Dielectrics (BL Dielectrics)", Review of the Electrical Communication Laboratories, 19:665 (1971) which discloses representative materials and processes using temperatures above 1000.degree. C. for both first and second firing stage for fabricating these dielectrics.
In recent years the term glass-ceramic has been used to describe, (1) a mixture of a low melting temperature glass and a high melting temperature ceramic which coalesces to a dense partially crystalline mass on heating; and (2) a glass composition obtained by rapid cooling which on subsequent heating usually to temperatures below 1000.degree. C. coalesces to a dense mass prior to crystallizing thereby resulting in a partially or wholly crystalline material. These glass ceramic materials have been fabricated as low dielectric constant materials as well as moderately high dielectric constant materials. Reference is made to Herczog, "Barrier Layers in Semiconducting Barium Titanate Glass-Ceramics", Journal of the American Ceramic Society, 67:484 (1984), and "Microcrystalline BaTiO.sub.3 by Crystallization from Glass", Journal of the American Ceramic Society, 47:107 (1964) which disclose a representative two-step firing process wherein interconnected dispersions of semiconducting BaTiO.sub.3 crystallites are sealed in a mostly glassy silicate matrix. However, there are several difficult problems associated with the oxidation or glass impregnation processes as described by Waku et al to produce GBBL capacitors: ( 1) the distribution of the insulating phase is uneven along the grain boundaries, (2) the thickness, 1-10 .mu.m is difficult to control, and (3) coarse-grained, on the order of 50 .mu.m, materials obtained from such processes are difficult to avoid. The first two of these problems contribute to lowering the electrical breakdown strength and hence the capacitor's reliability. Such breakdown could be caused by electrical short-circuiting due to conducting grain-to-grain contact or by dielectric instability due to a too thin intergranular layer. With respect to coarse-grained materials, such structures hinder the fabrication of GBBL capacitors using thin sheets (less than 1 mil) to achieve ultra-high capacity (mF/cm.sup.3)
The titanate glass ceramics as described by Herczog exhibit dielectric properties of mixed phase materials with an optimum dielectric constant of 1200. This value is not high enough for high capacity applications such as required for miniature decoupling capacitors in computer systems. Furthermore, if these materials are heated in a reducing atmosphere, they become semiconducting, indicating the presence of the interconnected BaTiO.sub.3 phase. They have expansion coefficients similar to BaTiO.sub.3 and cannot be joined or integrated into a substrate with expansion coefficient matching that of silicon.
GBBL ceramic materials as opposed to glass-ceramic materials are known to use barium titanate (BaTiO.sub.3) with different donor dopants in forming various types of ceramic capacitors having high capacitance. For example, U.S. Pat. No. 4,403,236 (Mandai et al) discloses boundary layer semiconducting ceramic capacitors comprising a semiconducting body in which grain boundaries on crystalline grains of the semiconducting ceramic body are made into an insulator. The ceramic composition consists of a large amount of a main component (Sr/Ba)TiO.sub.3, which may be modified with another titanate or a zirconate, and at least one semiconductor doping agent such as Nb, and Mn. The composition may also contain an oxide such as SiO.sub.2 and/or Al.sub.2 O.sub.3. The grain boundary of the crystalline ceramic body may be made insulating by heat-treatment in an oxidizing atmosphere to diffuse an insulating agent such as an oxidizable metal or a metal compound into the boundaries. The ceramic body itself may be produced by mixing the raw materials, forming the mixture into shaped bodies and then firing the shaped bodies in a neutral or reducing atmosphere. The grain sizes of the ceramic body may be up to 250 .mu.m.
U.S. Pat. No. Re. 29,484 (Utsumi et al) discloses a ceramic composition containing BaTiO.sub.3 as the basic composition which may also contain from 0.1 to 10 mol% of Nb.sub.2 O.sub.5 and other oxides, including Al.sub.2 O.sub.3. These materials are high dielectric constant materials and may be used as ceramic capacitors. The materials are formed by admixing the raw materials in a ball mill, pre-sintering, further ball mill mixing, pressure-molding into disks and then sintering.
U.S. Pat. No. 4,096,098 (Uneya et al) also describes a semiconductor ceramic composition containing barium titanate having a certain amount of the barium substituted with lead and calcium, and a semiconductor-forming component such as Nb. The ceramic compositions are prepared by conventional ceramic processes, e.g., mixing the powdered components following by sintering. The compositions are useful as heating elements.
U.S. Pat. No. 4,283,753 (Burn) discloses a ceramic capacitor composed of a dielectric ceramic body having a high-temperature-firing granular barium titanate phase and a low melting intergranular phase. The ceramic body may contain a number of ions of differing valencies, including Nb, which enter the lattice on titanium sites. Monolithic capacitors comprising a laminar assembly of ceramic layers of the disclosed dielectric material and having one or more buried electrodes are also disclosed.
U.S. Pat. No. 4,535,064 (Yoneda) relates to a barium titanate ceramic composition having a high dielectric constant useful as a reduction-reoxidation type semiconducting capacitor. A ceramic compound such as BaTiO.sub.3 is heated with an oxide of manganese and subjected to a reducing atmosphere to convert it to a semiconductor. Then, the semiconductor is heated in an oxidizing atmosphere to form a dielectric reoxidized layer on the surface of the semiconductor prior to application of electrodes. The ceramic compositions are stated to have a dielectric constant of as high as 15,000 and a grain size of as small as 1.0 to 1.5 .mu.m.
U.S. Pat. No. 4,606,116 (Hennings et al) discloses a non-linear resistor having a ceramic sintered body based on a polycrystalline alkaline earth metal titanate doped with a metal oxide to produce an n-type conductivity wherein the body has electrodes located on oppositely located surfaces. The ceramic body is first sintered in a reducing atmosphere to make the sintered body semiconductive, then the grain boundary layers of the semiconductor grains of the polycrystalline grain structure are converted by the formation of high-ohmic oxide layers by re-oxidation.
U.S. Pat. Nos. 4,405,475, 4,405,478 and 4,405,480 (all to Murase et al) describe dielectric ceramic materials composed of primary and secondary ingredients which form a polycrystalline ceramic structure, and insulating substances diffused throughout the intergranular boundaries of the ceramic to increase relative dielectric constant. The primary ingredients of these boundary layer ceramics are SrTiO.sub.3, Nb.sub.2 O.sub.5 and ZnO, and the secondary ingredients include silica and alumina, which serve to make the crystal grains of the ceramics larger in size, i.e., from 60 to 120 .mu.m. The insulating substances include oxides of lead, bismuth and boron.
U.S. Pat. No. 4,384,989 (Kamigaito et al) discloses a ceramic composition containing a semiconductive barium titanate, a doping element such as niobium, and an additive.
U.S. Pat. No. 4,362,637 (Matsuo et al) discloses grain boundary dielectric ceramic compositions useful as capacitors. The starting materials for producing these composition comprise SrO, TiO.sub.2 and Nb.sub.2 O.sub.5 and may contain other oxides.
The above patents disclosing variously constituted ceramic materials are manufactured by conventional ceramic processes, utilizing principles of densifying the crystallite materials by heating to sinter the crystallites. The sintering of the crystallites usually requires solid state diffusion, usually observed at high temperatures and accompanied by growth in the crystallite grain sizes.
Foss et al, in "Discrete Sedimented Decoupling Capacitor", IBM Technical Disclosure Bulletin, 26:1086-1087 (1983), disclose decoupling capacitors which may be mounted on an alumina or other packaging substrate bearing the decoupling capacitor as a thin firm high dielectric constant material. Barium titanate and high dielectric constant glass-ceramics are disclosed as suitable dielectric materials for the decoupling capacitor.
Crowder et al, in "Embedded Decoupling Capacitors For The Transverse Via Module", IBM Technical Disclosure Bulletin, 27:1556-1557 (1984) disclose eliminating discrete card or module chip circuit decoupling capacitors and integrating the capacitor as part of the substrate in a transverse via module for carrying semiconductor chips. This article also suggests integrating high dielectric layers (e.g., barium titanate) with standard ceramic layers in semiconductor chip packaging to improve effective decoupling capacitance.
U.S. Pat. No. 3,977,887 (McIntosh) discloses a high dielectric constant ceramic composition containing one or more polycrystalline materials (e.g., barium titanate and blends thereof such as bismuthniobate-barium titanate) and interstitial glass as a second essential component. The ceramic compositions are useful in multi-layer ceramic composites, and can be densified at relatively low sintering temperatures. The interstitial glass is a lead silicate based glass, essentially a ceramic frit, and forms an amorphous phase bonding the polycrystalline ceramic skeleton together, and also serving as a fluxing agent (liquid phase sintering) that permits the dense ceramic particles to be sintered at low temperatures. The compositions can be used as decoupling capacitors in multi-layer composite applications.
U.S. Pat. No. 4,234,367 (Herron et al) discloses glass-ceramic substrate carriers for mounting semiconductor or integrated circuit chips, including multi-layer substrates comprising a sintered glass-ceramic insulator and copper-based conductor pattern. These materials are useful for packaging semiconductor integrated devices and other elements. A number of metal oxides are disclosed as suitable crystallizable glass particles, including .beta.-spodumene or .alpha.-cordierite glasses. The particular process disclosed for making these materials includes an oxidation step which aids in removing polymeric binders from the multi-layer substrates.
U.S. Pat. No. 4,328,530 (Bajorek et al) discloses high capacitance semiconductor chip packaging structures which comprise an integrated ceramic carrier. Each of the conductive planes is composed of a single metallic sheet, sandwiched in the substrate with dielectric ceramic sheets to form stacked capacitor elements. The planar substrate comprises a stack of laminated ceramic sheets. The dielectric layers of the packaging structure may be composed of polyimide, glass, etc., and higher capacitances can be obtained by using glass-ceramic materials instead of pure alumina or mixtures of ceramics with ferroelectric materials or other high dielectric constant oxides.
The novel glass-ceramic material of the present invention described in detail hereinafter is particularly useful in chip packaging structures such as described in this '530 patent. In particular, the glass-ceramic materials of the present invention can be used within such a ceramic semiconductor chip packaging substrate to form a decoupling capacitor when placed between adjacent planes of conducting metal patterns, in particular between power planes of the multilayer substrate, and any number or all of the ceramic substrate layers or other high dielectric layers can be comprised of the material of the present invention. Thus, the disclosure of U.S. Pat. No. 4,328,530 is incorporated by reference herein, and will be referred to hereinafter with respect to specific embodiments.
U.S. Pat. Nos. 3,619,744 (Stephenson), 4,081,857 (Hanold, III), 4,086,649 (Hanold, III), 4,148,853 (Schuber), 4,158,219 (Payne et al), 4,459,364 (McSweeney et al), 4,469,747 (Sasaki et al), 4,528,613 (Stetson et al), and 4,616,289 (Itakura et al) all disclose types of multi-layer or monolithic ceramic capacitors employing barium titanate as a component.
Accordingly, due to the above-noted problems associated with known glass-ceramic and GBBL capacitors, room for improvement clearly exists, particularly in various practical applications employing capacitors.